My Raspberry Pi 2 boots successfully from the Raspbian (jessie) SD card, and the GUI seems to work properly. However, I have not been successful with the RISC OS: I bought an appropriate SD card, and as far as I can tell from the flickering of the ACT diode the system is booting correctly, but there is no video, and my monitor complains of 'no signal'.
I am using a VGA monitor with an HDMI-VGA converter, and a wireless mouse and keyboard. These work well with Raspbian, as does the ethernet cable, which seems to suggest that the solution lies elsewhere. But I guess that could be wrong. I've tried disconnecting the dongles and the ethernet cable, but there is still no video signal on boot-up.
The power diode glows a healthy red throughout, so I assume that is not the source of the problem.
Can anyone help?